1. Field of the Invention
The present invention generally relates to microwave amplifiers and, more particularly, to a microwave amplifier implemented by a transistor for amplifying signals having a millimeter-wave frequency or a microwave frequency.
2. Description of the Related Art
FIG. 4 is a circuit diagram of a microwave amplifier according to the related art disclosed, for example, in Japanese Laid-Open Patent Application No. 61-285811. Referring to FIG. 4, the microwave amplifier comprises a field-effect transistor 1 having a relatively high operating frequency, a grounding terminal 2 for the field-effect transistor 1, a stabilized resistor 3 connected to the grounding terminal 2, a grounding conductor pattern 4 having one end thereof connected to the stabilized resistor 3 and the other end grounded, and an open-circuit stub 5 having a length equal to 1/4 of a wavelength at the operating frequency of the field-effect transistor 1 and connected to the grounding terminal 2 so as to be parallel with a series circuit formed of the stabilized resistor 3 and the grounding conductor pattern 4.
A description will now be given of the operation of the microwave amplifier of FIG. 4.
The microwave amplifier shown in FIG. 4 operates such that a drain current from the drain D to the source S of the field-effect transistor 1 is amplified in accordance with a gate voltage applied to the gate G.
The grounding conductor pattern 4 is a channel by which the grounding terminal 2 of the field-effect transistor 1 is grounded. In a low-frequency band, the inductance of the grounding conductor pattern 4 is negligible so that the field-effect transistor 1 is properly grounded. In a high-frequency band, the inductance of the grounding conductor pattern 4 is not negligible. The grounding conductor pattern 4 thus acts as a short-circuited stub having inductance, resulting in a loss of the gain of the field-effect transistor 1 due to the inductance of the grounding conductor pattern 4.
Accordingly, the open-circuit stub 5 having a length equal to 1/4 of the wavelength at the operating frequency of the microwave amplifier is connected to the grounding terminal 2 to provide high-frequency grounding of the grounding terminal 2 at the operating frequency.
However, at certain points in the high-frequency band including the operating frequency, the inductance of the grounding conductor pattern 4 and the capacitance of the open-circuit stub 5 produce parallel resonance, causing the reactance of the grounding terminal 2 to become infinity at the parallel resonance frequency.
The stabilized resistor 3 is connected between the grounding terminal 2 and the grounding conductor pattern 4 in order to suppress parallel resonance at the parallel resonance frequency.
FIG. 5 is a circuit diagram of a microwave amplifier according to the related art shown in TECHNICAL REPORT OF IEICE (THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS) MW 92-149 LOW-NOISE AMPLFIER USING DIRECTLY COOLED HEMTs, FEBRUARY, 1993. Referring to FIG. 5, the microwave amplifier includes the field-effect transistor 1, the grounding terminal 2 of the field-effect transistor 1, and the inductor 7 having one end thereof connected to the grounding terminal 2 and the other end grounded.
A description will now be given of the operation of the microwave amplifier of FIG. 5.
The microwave amplifier shown in FIG. 5 operates such that a drain current from the drain D to the source S of the field-effect transistor 1 is amplified in accordance with a gate voltage applied to the gate G.
The inductor 7 can be configured such that the impedance that minimizes the noise for the field-effect transistor 1 substantially matches the impedance that minimizes reflection. Thus, the noise characteristic and the reflection characteristic can be simultaneously improved.
The microwave amplifier of FIG. 4 has a drawback in that the stabilized resistor 3 produces a voltage drop when a bias is applied to the field-effect transistor 1. This makes it difficult to use the construction as shown in FIG. 4 in a high-power amplifier having a large current consumption.
The microwave amplifier of FIG. 5 has a drawback in that when a member having inductance and capacitance is used to implement the inductor 7, parallel resonance results at a certain frequency (parallel resonance frequency) so that the reactance of the grounding terminal 2 becomes infinity at the parallel resonance frequency. If this occurs, the operation of the microwave amplifier becomes unstable.